Regeneration of spent aqueous solutions of caustic alkali containing mercaptides



Jan. 14, 1941. D. YABROFF 2,228,295

REGENERATION OF SPENT AQUEOUS SOLUTIONS OF CAUSTIC ALKALI CONTAININGMERCAPTIDES Filed Nov. 25, 1959 fiaso/lhe Lager 5k/mmed Off Waferfiolufion /nven1'or.- David L. Viz? By his Affarneg Patented Jan. 14,1941 PATENT OFFICE REGENERATION F SPENT AQUEOUS SO- LUTIONS 0F CAUSTICALKALI CONTAIN- ING MERCAPTIDES David Louis Yabroll', Berkeley, Calif.,assignor to Shell Development Company, San Francisco, Calif., acorporation of Delaware Application November 25, 1939, Serial No.306,148

6 Claims. (Cl. 260-609) This invention relates to the regeneration ofspent aqueous solutions of caustic alkali containing absorbedmercaptides and solutizers for mercaptans, and more particularly isconcerned with a simple pretreating step preceding the regenerationproper.

It is known that sour hydrocarbon distillates and, more particularly,gasoline distillates can be sweetened by extracting mercaptans contained10 therein with aqueous solutions of alkali metal hydroxides containingsolutizers. In this process, the 'role of the solutizer is, to increasethe solvent power of the aqueous solution for free mercaptans.solutizers, therefore, are substances which are readily soluble inaqueous solutions of caustic alkali, are substantially insoluble in thehydrocarbon oil to be extracted, are good solvents for free mercaptans,are inert to the caustic alkali even at the elevated temperatures ofsteam strip- 20. ping, and preferably have boiling'temperaturessubstantially above that of water. To be effective, the solutizer mustbe present in an amount sumcient to materially raise the solvent powerof the aqueous solution for mercaptans, e. g., normally inamounts inexcess of about 15%.

Among the many substances having solutizing properties, the followingare the most useful: Amino and hydroxy alkyl amines in which the alkylradicals have 2 to 3 carbon atoms; glycols, amino glycols and diaminoalcohols of 3 to 5 carbon atoms; diamino, dihydroxy or amino hydroxydialkyl ethers or thioethers in which the alkyl radicals have 2 to 3carbon atoms; alkali metal, and particularly potassium salts of fattyacids having from 3 to 5 carbon atoms, or of hydroxy or amino fattyacids having from 4 to 7 carbon atoms, or of phenyl acetic or hydroxy oramino phenyl acetic acids, or of alkyl phenols; or mixtures of theabove. V

More detailed information about the various solutizers and the amountsin which they are effective has been disclosed in U. S. Patents2,152,720, 2,152,723, 2,152,166, 2,149,379, 2,149,380, 2,156,577,2,152,722, 2,164,851 and Serial Nos. 255,100 (filed February '7, 1939)and 271,962 (filed May 5, 1939).

It has been found that even though the solutizers are substantiallyinsoluble in hydrocarbon oils, their presence in caustic alkalisolutions in amounts sufiicient materially to raise the solvent power ofthese solutions for mercaptans causes dissolution of appreciable amountsof the hydrocarbon oils in the aqueous phase when extracting the oilswith the 'solutized solution. Thereafter, 55 when regenerating spentextracting solution con- .volume hydrocarbons. Upon the addition ofwataining mercaptides and the dissolved oil, as by steam stripping,hydrocarbon oil and mercaptans are expelled together. Upon condensationof the resulting overhead vapors, a condensate is obtained whichsegregates into two layers, a water layer and an oily mercaptan layer,the latter being highly contaminated with hydrocarbon oil. Frequently itis desired to produce relatively pure mercaptans and the content ofhydrocarbon oil in the mercaptan layer is, therefore, of distinctdisadvantage. The mercaptans and hydrocarbons cannot be separated bydistillation, since they usually have similar boiling ranges. Analysesof mercaptan layers have shown that they sometimes contain 80% and morehydrocarbons.

Now I have discovered that when diluting with Water a spent aqueoussolution containing an effective amount of solutizer, absorbedmercaptansand hydrocarbons, a portion of the oil is released and separates out,without a simultaneous release of a proportionate amount of the absorbedmercaptans. The separated oil may be removed as by settling,centrifuging or the like. While it is usually sour, its content ofmercaptans is P not normally much greater than that of the hydrocarbonoil from which it had been extracted. Hence, if desired, it may bereturned to the sour distillate receiving treatment, or it may be usedfor any other desired purpose. Q0

The diluted aqueous caustic alkali solution freed from a large portionof the absorbed hydrocarbon oil may then be regenerated by steamstripping in the usual way and preferably by the method described in theYabrofi-White Patent 2,152,724. Mercaptans thus recovered, thoughnormally still containing a small amount of neutral oil, are of a muchhigherdegree of purity than prior to the separating step described.Whereas prior to the separating step, contents of 80% and more neutraloil are the rule, after the separating step this content is frequentlybelow 15% and occasionally even below 5%.

The percent of dissolved oil separated depends primarily on the degreeof dilution. The following example shows the effect which dilution withvarious amounts of water may have on the solubility of hydrocarbons inthe solutized aqueous caustic alkali solution.

A sour gasoline was extracted with an aqueous alkaline solutioncontaining 6 gm. mol KOH and 3 gm. mol potassium isobutyrate per liter.The spent solution was analyzed for its hydrocarbon oil content, and wasfound to contain .45% by ter, the content of hydrocarbon decreased as Myprocess is further illustrated in the attached drawing which representsa simplified flow diagram of a preferred form of my process.

A sour gasoline distillate is introducedfrom a source notshown intoextractor I through line 2, and is sweetened by extraction with anaqueous solution of a caustic alkali containing sumcient solutizer toenable the desired sweetening to take place. Sweet distillate emergesthrough line 3 and fat aqueous solution containing ab-.

sorbed 'mercaptans and hydrocarbons passes through bottom line 4 to asurge or settling tank 5. Water recovered from the stripping operationin. a manner presently to be described is added from line 6. to the fatsolution in line 4. Immediately upon dilution, the fat solution releasesa port on of its dissolved hydrocarbons, which in tank& is allowed torise to the top and is withdrawn through line I. If desired, theseparated oil may be returned through line 8 to the sour distillateentering extractor. I

The resulting lean solution, which still contains substantially all ofthe absorbed mercaptans passes from tank 5 through heat exchanger 9 inline H, and the resulting hot solution enters steam stripper i2 at apoint immediately below tray i3. Tray l3 divides the stripper I 2 intoan upper smaller reflux section and a lower large stripping section. Thehot lean solution flows down through the stripper section incountercurrent to stripping steam which enters the stripper near itsbottom through line H. The stripped lean-solution then passes from thestripper l2 through line I 5 to reboiler l6, where the water ofdilution, which had been added to the fat solution in line B, isvaporized, and steam so produced is used for stripping, returning to thesteam stripper through line H.

Regenerated and reconcentrated caustic alkali solution returns to thetop of extractor I through line ll to sweeten further amounts oi sourdistillate.

The steam in stripper l2 rises through plate I3 into the upper refluxsection in which a portion of the small amount of residual dissolvedhydrocarbons, which have remained in the lean solution after dilutionand which are'expelled in the stripper, may be condensed and returned totank 5 together with the water of dilution, through lines l8 and 6 andheat exchanger 9.

The small amount of mercaptans dissolved in the reflux added throughline 22 to stripper 12 are steamed out in the reflux section (aboveplate i3) and the water of dilution flowing through line [8 is thussubstantially free of mercaptan. This reflux also serves as a spraycatcher.

The remaining vapors emerge from the reflux section of stripper l2through line l9 and are condensed in condenser 20. The resultingcondensate flows into receiving tank 2|, where two liquid layers areformed, an upper relatively pure mercaptan layer which is withdrawnthrough line 23, and a lower water layer, containing small amounts ofmercaptans, which layer is tially free from mercaptans.

returned'through line 22 to the stripping sections where it serves asreflux. In the course of the refluxing. the small amounts of mercaptanscontained in the. reflux water are revaporized so'that the waterwithdrawn from plate I3 and used for dilution purposes in tank 5 issubstan- The amount or. water available for dilution in tank 5, in acompletely closed system as the one described above, is controlled bythe amount oi! steam required to effect the regeneration. The more steamis used for stripping, the more water is available for diluting.Experience has shown that the amount oi steam required varies betweenwide limits. depending not only on the nature of the hydrocarbondistillate to be sweet- -ened, but also on the number or theoreticalstages in both the extraction and stripping equipment available. Thus,the amount of water available for dilution may normally vary betweenabout 5% to 200% by volumeor the spent aqueous caustic alkali solution.

Even in a system which is not completely closed, 1. e., in which waterfor dilution may be introduced into line 6 and tank I from an outsidesource through line 24, in which further excessive water of condensationin. tank 2| may be disposed of through line 25. and in which steam forstripping may be introduced from an outside source through line 26, theamount of water of dilution may not exceed that which is removed byevaporation in the reboiler l6, otherwise the circulating solution wouldbecome more and more diluted and incapable of sweetening the sourdistillate.

' I claim as my invention:

1. In the process of regenerating by steam stripping a spent aqueoussolution of caustic alkali which contains an amount of solutizersuflioient materially to raise the solvent power of the aqueous solutionfor mercaptans, which further contains mercaptans and. dissolvedhydrocarbon oil, and which has been used to extract mercaptans from sourhydrocarbon 'distillates, the improvement comprising diluting said spentsolution with water, whereby at least a portion of said dissolved oil isprecipitated to form a separate phase, separating the precipitated oilfrom the diluted solution and thereafter subjecting the latter to steamstripping to expel mercaptans.

2. In the process of regenerating by steam stripping a spent aqueoussolution of caustic alkali which contains an amount of solutizersuflicient materially to raise the solvent power of the aqueous solutionfor mercaptans, which further contains mercaptans and dissolvedhydrocarbon oil, and which has been used to extract mercaptans from sourhydrocarbon distillates, the improvement comprising diluting said spentsolution with an amount by weight of water not greater than the amountby weight of steam required in the steam stripping, whereby at least aportion of said dissolved oil is precipitated to Y form a separatephase, separating the precipitated oil from the diluted solution, and-thereafter stripping the latter with said amount of steam.

3. The process of claim 2 in which the amount,

of water of dilution is between 5% and 200% of the spent aqueoussolution.

further contains mercaptans and dissolved hydrocarbon oil, and which hasbeen used to extract mercaptans from sour hydrocarbon distillates, theimprovement comprising diluting said spent solution with an amount byweight or water substantially equal to the amount by weight of steamrequired in the steam stripping, whereby at least a portion of saiddissolved oil is precipitated to form a separate phase, separating theprecipitated oil from the diluted solution, and thereafter stripping thelatter with said amount or steam.

5. In the process of regenerating by steam stripping a spent aqueoussolution of caustic alkali which contains an amount of solutizersufficient materially to raise the solvent power of the aqueous solutionfor mercaptans, which further contains mercaptans and dissolved hydro.carbon oil, and which has been used to extract mercaptans from sourhydrocarbon distillates, the improvement comprising diluting said spentsolution with water, whereby at least a portion of said dissolved oil isprecipitated to form a separate phase, separating the precipitated oilfrom the diluted solution, stripping the latter with steam to removemercaptans, thereby producing a foul steam containing mercaptans,condensing the foul steam to form a condensate which separates into twoliquid layers, an upper 30 mercaptan layer and a lower aqueous layer,

separating the layers and using at least a portion of the condensedwater for diluting said spent solution.

6. In the process of regenerating by steam stripping a spent aqueoussolution of caustic alkali which contains an amount of solutizersumcient materially to raise the solventpower of the aqueous solutionfor mercaptans, which further contains mercaptans and dissolvedhydrocarbon oil, and which has been used to extract mercaptans from sourhydrocarbon distillates, the improvement comprising diluting said spentsolution with water, whereby at least a portion or said dissolved oil isprecipitated to form a separate phase, separating the precipitated oilfrom the diluted solution, stripping the latter with steam to removemercaptans, thereby producing a foul steam containing mercaptans,refluxing said foul steam with water, condensing the refluxed foul steamto form a condensate which separates into two layers, an upper mercaptanlayer and a lower water layer containing small amounts of mercaptans,separating the layers, using at least a portion of said water layer forsaid refluxing of said 'foul steam, whereby mercaptans dissolved in saidportion are vaporized, and further using the resulting portionsubstantially free from mercaptans for diluting said spent solution.

DAVID LOUIS YABROFF.

